In the following discussion, the feeding systems currently being used to feed low-pressure slurry into a digester operating at high pressure will be briefly described with reference to a number of patents disclosing the known devices and processes in more detail. Also, the problems related to the present feeders, to their construction or to the use of these feeders, will be discussed.
The continuous pulping process was developed in the 1940s and 1950s. Since then, no dramatic improvements have been made to the equipment for transferring the comminuted cellulosic material to the digester. The High-Pressure Feeder (HPF) has been used for decades for feeding a slurry of wood chips into the treatment vessel, and it still seems to be an object of further developments. The HPF is a rotary valve-type device, which transfers the slurry of material and liquid at one pressure to a second, higher pressure. The transfer is performed with the aid of circulation pumps. One advantage of the HPF is its capability of acting as a pressure isolation valve, by preventing the high-pressure material from escaping to the low-pressure side, or to the surrounding environment.
Although the main concept of the HPF has remained unchanged during these years the development of the feeder has been continuous. There are numerous patents describing different progress steps of HPF, for example the following: U.S. Pat. Nos. 2,459,180; 2,688,416; 2,870,009; 2,901,149; 2,914,223; 3,041,232; 4,033,811; 4,338,049; 4,430,029; 4,508,473 and 4,516,887. U.S. Pat. Nos. 5,236,285 and 5,236,286 describe the design of the High-Pressure Feeder in the mid 1990s. Examples of recent developments are disclosed in U.S. Pat. Nos. 6,468,006, 6,616,384 and 6,669,410.
In a continuous digesting system the method for feeding comminuted cellulosic material into the pressurized treatment vessel has proved to be a very demanding part of the process. The HPF with only relatively few substantive changes in its design has been the central part of the feeding system of the Kamyr continuous pulping system. It seems to have been difficult, if not impossible, to solve the bottlenecks of the HPF, for example the low density of bulk material inside the feeder. Attempts have been made to enhance the efficiency by developing the method of chip feeding as a whole. The method, marketed under the name LO-LEVEL® Feed System, by Andritz simplifies the system around the HPF. This feed system is described for example in the following U.S. Pat. Nos. 5,476,572; 5,622,598; 5,635,025; 5,736,006; 5,753,075; 5,766,418 and 5,795,438. Recent developments further simplify the equipment related to the feeding system by eliminating the need for a separate liquor storage vessel and a separate level controlling vessel or tank. This system with a single tank is described in the U.S. Pat. No. 6,368,453, in a divisional application US2001/0025694A1 and in its five other divisional applications.
One bottleneck in the present High Pressure Feeders is the reduction in the pocket cross-sectional area in the middle of the pocket as a consequence of the crosswise placed pockets within the rotor. Due to this reduction of the pockets more flushing is needed to avoid plugging.
The design of the present high-pressure feeder is conical, in terms of both the rotor and the housing. When the wear parts have to be repaired the wear surfaces have to be weld repaired and machined afterwards.
There are typically four pockets per revolution in the present high-pressure feeder so the filling and emptying frequency is low, meaning that the rotating frequency is quite low (maximum 15-18 rpm) and therefore the specific oscillating frequency is low and differs only slightly from the specific oscillating frequency of the building itself, resulting in a wobble phenomenon. The low rotating frequency also reduces the capacity of the feeder.
Also, the manually adjusted seal gap between the housing and the rotor contributes to leakage and wear problems with the HPF.
The manufacturing costs of present high-pressure feeders are high in relation to the capacity obtained. The construction of the pockets is complicated and the feeder construction as a whole has to be very rigid due to asymmetrical loads on the rotor during the filling and emptying of the pockets, both of which increase the manufacturing costs.
In the mid 1990's Kvaerner introduced a feeding system, disclosed in U.S. Pat. No. 5,744,004, that was not based on the use of a feeder operating as a sluice between the low pressure and high pressure sides. Instead, a pump or a series of pumps was used, which comprises a stack, or so called disc pack, consisting of a number of parallel discs held together and rotating in a pump housing about a common axis of rotation. One of the pumps coupled in series is arranged so that it can be rotated with a variable speed of rotation in order to regulate the pressure in the digester. These pumps are known under the trade name DISCFLO.
In U.S. Pat. No. 3,758,379, a type of revolver feeder is described, for example, for the handling of cellulose-containing material. The feeder according to that invention comprises a rotor having a plurality of spaces extending axially through the rotor in different positions of the rotor, which spaces are brought into communication with openings formed in end plates contacting the rotor. The feeder comprises a feed end plate and a discharge end plate, which are interconnected by drawbars equally spaced about the circumference of the end plates outside the rotor. According to the specification of U.S. Pat. No. 3,758,379, the unbalanced pressure forces acting on the end plates cause a problem typical to these revolver-type feeder valves. These forces cause rapid and irregular wear of the opposing surfaces of the rotor and the end plates, which results in leakage between the spaces. According to U.S. Pat. No. 3,758,379 that problem is solved, in other words an essentially constant and uniform wear of the sealing surfaces is obtained, by using traction force producing devices disposed individually for all or those draw bars positioned adjacent the areas exposed to high outwardly directed pressure forces from the spaces. These traction force producing devices balance the pressure forces acting on the end plates and render the movement of the end plates uniform in relation to the ends of the rotor along the circumference of the rotor. According to this invention, each of the traction force-producing devices contains an automatically operative pressure control device to produce a traction force to counterbalance the increased pressure directed outwardly from the spaces of the rotor. To obtain the balanced contact, removable sealing discs are placed between the opposing faces of the rotor and the end plates. This type of feeder is very complicated and also expensive.
It is one of the objects of the present invention to provide a new transfer device for transferring bulk material from one pressure level system to another pressure level system, especially to provide a transfer device for feeding wood chips into a digester, a feeder, that transfers low-pressure slurry from the chip chute to the digester operating at high pressure. The above-described problems associated with prior art feeding systems can be solved with this new type of a feeder.